Characteristics of the basement reformed volcanic edifice in Bohai Sea and its implication for hydrocarbon enrichment

The intact volcanic edifice had been formed and finalized during the depositional stage of Mesozoic Yixian Formation in the Bohai Sea. The residual body of the intact volcanic edifice after reformation was defined as the basement reformed volcanic edifice. Based on drilling, seismic, well logging and microscopic data in the study area, characteristics of the basement reformed volcanic edifice were well discussed, and its control on oil and gas was also investigated. Results showed that the basement reformed volcanic edifice was jointly controlled by magmatic property, paleo-geomorphology and late reformation, and the late fault cutting and differential denudation were major reformation ways of the volcanic edifice; strong and weak volcanic eruption patterns developed in the study belonged to the inverted-sequence eruption, and the explosive facies and the intermediate-acidic or acidic effusive facies were the favorable reservoir facies; weathering, cycles and period boundaries controlled vertical distribution of high-quality lithofacies, and coupling between each periodic interface at different levels and residual high-quality lithofacies was the key to oil and gas enrichment. Major exploration targets of volcanic rocks were to find periodic interfaces in the volcanic edifice with the residual high-quality lithofacies within 150 m from the weathering crust or eruption surface.